#include <iostream>
#include <unordered_set>
using namespace std;

int main()
{
	int n;
	cin>>n;
	string s;
	cin>>s;
	int ret=0;
	unordered_set<char> has;
	for(int i=0;i<n;i++)
	{
		if(has.count(s[i]))
			continue;
		has.insert(s[i]);
		ret++;
	}
	cout<<ret<<endl;
	return 0;
}
